Cloth sim : collision with external linked group meshes
I just tried something else, which works... (with Blender 2.67b, however)

I created a new empty blend file, and linked both the character AND the scarf into it. I had to re-configure the cloth settings of the scarf because it seems these ones aren't linkable, but worked like a charm. And since it's the final "workflow" we're looking for (i.e. linking both the scarf and the character, into another "scene" .blend file), it's great !

But, it might be a bug of the 2.67 (before "bugfixes" releases)...